Some of mine:
WCW:
The Four Horsemen: Whether it be the 7th version or the 8th version (the one thing the horse whinnieing in the beginning, it is perhaps the greatest entrance theme of all time, certainly one of the best. It makes the hair on the back of your neck stand up when that opening rift hits.
DDP: Yeah, it's a rip off of Smells Like Teen Spirit, but it's still great.
Booker T/Harlem Heat: Extremely catchy, great beat, and I like all the funny noises and weird laughing in it. lol
Alex Wright: You can't help but dance any time you hear it, just not the Alex Wright dance. LOL
Vampiro/Billy Kidman: Love the drums and guitars in it. I didn't the song fit Vampiro well. It wasn't gothy or anything, but still an excellent theme nonetheless.
Bret Hart: I don't remember his first two themes, but his third theme I found on YouTube is GREAT. If I had to pick a theme of his that ranks up there with his WWE one, this is it.
Oldschool WWF:
Jake the Snake Roberts: Both his face and heel themes are GREAT. His face theme is better, both fit him perfectly.
Hulk Hogan: Like I need to say why?
Bret Hart: Right up there with Jake the Snake as perhaps the greatest WWF theme ever.
Demolition: THE DEMOS WILL GET YOU!! 'nuff said.
Orient Express/Bull Nakano: I think almost every Japanese wrestler used this as their theme at some point. lol But it's still a great theme, albeit very repetitive.
Goldust: In the 90's he was one of the best superstars IMO, he had a great rivalry with Razor Ramon. But anyway, I thought his theme was great.